Add wxOVERRIDE and use it in common and wxOSX code.

Make overriding virtual methods more explicit and enable additional checks
provided by C++11 compilers when "override" is used.

Closes #16100.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76173 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/unix/appunix.cpp

@@ -53,7 +53,7 @@ public:
                                     );
     }
 
-    virtual void OnReadWaiting()
+    virtual void OnReadWaiting() wxOVERRIDE
     {
         // The base class wxWakeUpPipe::OnReadWaiting() needs to be called in order
         // to read the data out of the wake up pipe and clear it for next time.